The insurance sidecar is making a return to the London market, with Willis reportedly in the final stages of negotiating a facility to cover up to 20% of its specialty insurance book running through Lloyd’s.

It follows Aon Risk Solutions’ co-insurance deal with Berkshire Hathaway in March, which continues to provoke debate in London about the effect on Lloyd’s and whether the new capital attached to the sidecar will squeeze out market syndicates.

Berkshire Hathaway is also rumoured to be the partner in the Willis Global 360 facility.

The Aon sidecar will give retail clients benefits that are usually exclusive to reinsurance markets, Group President Steve McGill says.

Clients and the Lloyd’s market will gain access to Berkshire’s capital, while the Warren Buffett-led reinsurer will get a 7.5% share of Aon business brokered through Lloyd’s.

Mr McGill says it is the first sidecar of its kind. No other company has been able to deliver an offering of such scale.

Aon has responded to criticism from some London industry leaders by claiming the arrangement makes capital available to sectors that have traditionally been challenged.

While some observers praise sidecars for bringing extra capacity to the market, others regard the facilities as a way to undermine Lloyd’s and raise commission income.